DOWNBURSTS IN THE NORTHWESTERN PART OF SAITAMA PREFECTURE ON 8 SEPTEMBER 1994

On the afternoon of 8 September 1994, a severe thunderstorm passed ove r the Gunma and Saitama prefectures, north of Tokyo. It lasted for mor e than 3 hours and produced a strong gusty wind associated with hail. The wind produced damage to window glass of the Misato Junior High Sch ool (MJHS), injuring 2 teachers and 71 students. The damage-producing wind and its parent thunderstorm were analyzed by using data from a da mage survey, surface meteorological stations, upper air soundings, sat ellite images and a conventional radar. It is identified that at least three downbursts occurred in association with the storm. The storm mo ved toward the east-southeast direction at a speed of about 8 m s(-1). It was accompanied with a temperature drop of about 10 K and a diverg ent wind, though not initially of damage-producing intensity, below it s cloud base from its developing stage. Its cloud-top reached about 15 km AGL in its mature stage. The radar reflectivity data showed that i t had a marked overhang in the direction of the movement. The width of the region with damage-producing hail below the storm markedly increa sed twice. The times and locations of the increases coincided with tho se when and where descents of a reflectivity core were indicated from the radar. Furthermore, time-space-converted wind data at several surf ace stations showed that divergent wind fields, corresponding to Downb ursts A and C, occurred near the times and locations of these events. Downburst A occurred right in the middle of Kodama environmental monit oring station (KD) and Kodama District Fire Station which are only 3 k m apart, and was clearly delineated from the wind record at these two stations. The temperature drop due to the passage of the storm was lar gest near the two stations and was more than 11 K. The cool air origin ating from Downburst A later spread over a region of 40 km in diameter . After the occurrence of Downburst A, the storm quickly dissipated. D ownburst B occurred near KD, where another indication of a divergent w ind field and a corresponding further temperature drop were recorded. The portion of the storm where Downburst B was generated passed over M JHS at about the time of the damage-producing wind. Though no observat ional data exist near MJHS, it is conjectured from the damage survey a nd these facts that the fourth downburst may have occurred near MJHS. All of the downbursts occurred 6-10 km behind the gust front. CAFE of the storm environment decreased from more than 1800 m(-2) s(-2) to les s than 700 m(-2) s(-2) during 3 hours of the passage of the storm. The difference of equivalent potential temperature between the low- and m id-levels similarly decreased from more than 26 K to less than 16 K.